1) Field of the Invention
This invention relates to an optical disc system or magneto-optic disc system (hereinafter collectively called the "optical disc system" for the sake of simplicity), and specifically to an optical disc system designed to prevent noise while an optical disc is driven.
2) Description of the Related Art
An optical disc system of the above type is equipped with an optical pickup means for radiating an optical beam onto an optical disc, detecting any focusing error and tracking error (i.e., any tracing error for a desired track of the optical disc) from light reflected by the optical disc, and outputting a focusing error signal and a tracking error signal; a drive means for outputting a focusing drive current and a tracking current signal on the basis of the focusing error signal and tracking error signal, respectively; and an actuator for shifting the optical pickup means in focusing and tracking directions in accordance with the focusing drive current and tracking drive current, respectively.
In the above optical disc system, the optical disc is rotated by a spindle motor, and the actuator is operated by currents outputted from the drive means on the basis of the focusing error signal and the tracking error signal both outputted from the optical pickup means. Feedback (servo) control is performed by means of the actuator, whereby the optical pickup means is shifted in the focusing and tracking directions to eliminate the focusing and tracking errors. Information of the optical disc is read in this manner.
FIG. 8 is a perspective view of an optical disc, while FIG. 9 is an enlarged view of an index point (hereinafter called the "ID point") of the optical disc.
In the optical disc 1, address information such as track numbers and sector numbers are formatted beforehand at such ID points. Each ID point is located at the leading end of its corresponding sector and, as is shown in FIG. 9, contents such as the address of the corresponding data area are recorded by pits 2.
When an optical beams passes across the ID point, influence by the pits 2 appears in a focusing error signal. Namely, the optical pickup means detects the pits 2, and the detection of the pits 2 results in periodical focusing errors which take place whenever the pickup means passes across the ID points. These focusing errors cause the actuator to undergo minute mechanical vibrations.
FIG. 10 is a waveform diagram showing the waveforms of a focusing error signal AF and a focusing drive current FS. It is observed that their waveforms peak out at each place where an optical beam passes across an ID point. Each waveform section indicated by ".alpha." in FIG. 10 reflects the influence of pits 2 at an ID point, whereas each waveform section designated by ".beta." reflects a response to a focusing servo to the section .alpha..
The disturbance .beta. by this focusing servo occurs in every sector, and the period T(s) of the waveform .beta. can be expressed by the following formula (1): ##EQU1## where N: revolution number of the disc (rpm),
M: number of sectors. PA1 an optical pickup means for radiating an optical beam onto an optical disc with plural index points formed thereon, detecting any focusing error and any tracking error from light reflected by the optical disc, and then outputting a focusing error signal and a tracking error signal to a focusing servo control signal system and a tracking servo control system, respectively; PA1 a drive means for outputting a focusing drive current and a tracking drive current on the basis of the focusing error signal and tracking error signal, respectively; PA1 an actuator for shifting the optical pickup means in a focusing direction and in a tracking direction by the focusing drive current and tracking drive current, respectively; PA1 a signal detection and processing means for detecting each of the index points and generating an index gate signal indicating an address between the index point and another index point next to the first-mentioned index point in the tracking direction; and PA1 a holding means for holding, based on the index gate signal, the focusing error signal for a predetermined time at a signal level right before the first-mentioned index point.
On the other hand, the frequency f can be defined by the following formula (2): ##EQU2##
With the formula (2) in view, the frequency f when the revolution number and sector number are set, for example, at 3,600 rpm and 17, respectively, can be given by the following formula (3): ##EQU3##
Incidentally, the frequency f is proportional to the disc revolution number N as is easily understood from the formula (2). A reduction in the revolution number of the disk, therefore, makes it possible to reduce vibrations of the actuator to an extent such that they no longer cause noise detectable by the human ear. Optical disc systems driven at a low revolution number, therefore, do not cause a noise problem. However, the frequency f.apprxeq.1 KHz in the focusing direction as determined by the formula (3) causes the human ear to detect substantial noise substantial noise.
If the disc revolution number N is reduced in view of the effect of vibrations of the actuator to the human ear, the noise can be lowered. A reduction in the disc revolution number N, however, leads to a lowered data transfer rate (the term "data transfer rate" means the speed at which data recorded on the optical disc 10 are read and then transferred to a control unit or the speed at which writing data from the control unit are written on the optical disc 1), thereby making it impossible to maintain the performance as an optical disc system. It has hence been difficult to increase the disc revolution number N and, at the same time, reduce the noise.